The present invention relates to a wet press for a machine for the manufacture of paper, board or similar webs of fibrous material, of the type which comprises a press shell which passes continuously over a press shoe supported on a support member, and a mating roll which opposes the press shoe and forms a press nip with the press roll. More particularly, it is related to means for preventing, or at least reducing, the remoistening of the web of fibrous material upon its emergence from the nip of the wet press.
This problem can occur in wet pressed which have only a single water-absorbing endless belt (so-called single felt presses), but is particularly serious in those presses in which the web of fibrous material passes through the press nip between two belts, in which one of the two endless belts, hereinafter called at times the "first belt", always has a higher water-absorbing capacity than the other endless belt, hereinafter called at times the "second belt". Both belts can be felt belts; in such case the press is a so-called double-felt press.
The following publications disclose known presses which are in the background of the present invention:
1. U.S. Pat. No. 3,607,626, issued Sept. 21, 1971. PA1 2. Federal Republic of Germany OS No. 33 24 408, published Feb. 2, 1984. PA1 3. U.S. Pat. No. 4,503,765, issued Mar. 12, 1985, commonly assigned herewith, whose disclosures are incorporated by reference herein. PA1 4. German Democratic Republic Pat. No. 79,719 issued Feb. 12, 1971.
In the single-felt press of GDR Pat. No. 79,719, the web of fibrous material from which the water is to be removed passes through the press nip between a water-absorbing felt belt and a non-water-absorbing liquid-impervious press belt.
Federal Republic of Germany OS No. 3,324,408 suggests that in order to reduce the remoistening of the web of fibrous material it should be endeavored to separate the paper web from the two felt belts directly behind the press nip. Actual practice, however, shows that this desire is frequently not realizable, since in actual practice the web of fibrous material from which the water is to be removed always travels together with one of the two felt belts at least over a short joint path of travel beyond the press nip. To be sure, one can attempt to shorten the joint path by increasing the tensile stress in the discharging web of fibrous material. However, close limits exist a to what can be achieved, since the web of fibrous material which is still moist undergoes an undesired elongation under tensile stress, as a result of which, as a general rule, the quality of the final web of fibrous material is reduced.
The present invention is also concerned with the problems which relate to the so-called process of "licking-up" the web of fibrous material, i.e. with the threading of the web of fibrous material into the paper machine (upon the starting of the paper machine or after breakage of the web). During the lick-up, as it is also known, initially only a narrow strip at the edge of the web of fibrous material travels through the wet press, and from there to the next section of the paper machine. The web of fibrous material is then slowly brought to its full width with the tensile stress in the web being as a rule set at a particularly low value. This has the result that the web of fibrous material upon its emergence from the press nip of the wet press initially travels over a relatively long joint path of travel with the felt belt or one of the two felt belts. By a careful increase in the tensile stress, one then attempts to shorten the joint path of travel, so as to keep the remoistening of the web of fibrous material by the felt belt as slight as possible.
Another problem resides in the fact that the end of the joint path of travel, where the web of fibrous material detaches itself from the felt belt, is not precisely defined. In other words, the web of fibrous material sometime detaches itself earlier and sometimes after from the felt belt in question. This detachment furthermore does not always occur at the same place over the width of the paper machine. In a double-felt press it can also happen that a part of the web of fibrous material tends to travel with one of the felt belts and another part of the web of fibrous material with the other felt belt. As a result of all of these factors, the quality of the web of fibrous material may be impaired, and there may be a danger of the web of fibrous material being torn.
In U.S. Pat. No. 3,607,626, measures are described for reducing the remoistening of the web of fibrous material. From that patent it is known that the web of fibrous material from which the water is to be removed normally follows the wetter felt belt behind the nip of the double felt press, that is, the felt belt which has absorbed the larger amount of water from the web of fibrous material in the nip of the press. The web of fibrous material adheres more strongly to this felt belt than to the drier felt belt. As a result of this, the tendency towards remoistening of the web of fibrous material behind the press nip is particularly great. In order to counteract this tendency, measures are described in U.S. Pat. No. 3,607,626 which are intended to have the result that, in the normal operation of the paper machine, the web of fibrous material does not travel together with the wetter felt belt behind the nip of the press, but rather with the drier felt belt. For this purpose, a suction zone is arranged behind the press nip, in the press roll which the drier felt belt is wrapped around. Means are also provided for displacing the path of travel of the wetter felt belt so that during the licking-up of the web of paper it travels over the suction zone together with the drier felt belt. The suction zone holds the web of paper firm against the drier felt belt even when the wetter felt belt assumes its normal path of travel in continuous operation. However, this known arrangement has disadvantages, namely the expense of the suction zone means, and the remoistening of the web of fibrous material by the drier felt of the web, which is still present to some degree.